Open AccessThe P ‐wave upper mantle structure beneath an active spreading centre: the Gulf of California
Author(s) -
Walck Marianne c.
Publication year - 1984
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1984.tb01918.x
Subject(s) - geology , seismogram , mantle (geology) , seismology , amplitude , trench , transition zone , tectonics , ridge , geodesy , geophysics , paleontology , chemistry , physics , organic chemistry , layer (electronics) , quantum mechanics
Summary. Detailed analysis of short‐period travel‐time, dT / d δ and wave‐form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km. More than 1400 digital seismograms from earthquakes in Mexico and central America recorded at SCARLET yield 1753 travel times and 58 direct measurements of dT / d δ as well as high‐quality, stable waveforms. The 29 events combine to form a continuous record section from 9° to 40° with an average station spacing of less than 5 km. First the travel times are inverted using the tau method of Bessonova et al. ; the resultant model is adjusted to agree with the experimental p ‐δ values. Further constraints arise from the observed relative amplitudes of mantle phases, which are modelled by trial and error using WKBJ synthetic seismograms (Chapman; Wiggins). Model GCA, which is appropriate for western Mexico north of 20° latitude, is similar to existing upper mantle models for shield, tectonic‐continental, and arc‐trench regimes below 400 km, but differs significantly in the upper 350 km. GCA velocities are very low in this depth range; the model ‘catches up’ with the others with a very large velocity gradient from 225 to 390 km. This well‐resolved feature is consistent with the shear‐wave model TNA for western North America of Grand & Helmberger. The abundant data from 20° to 30° control the detailed shape of the 660‐km discontinuity. Very large velocity gradients lie both above (620‐660 km) and below (661‐680km) a 2.8 per cent velocity change.